21. Water management feature: Record the type(s) of water management
feature(s) nearest the segment and the distance from the reference location. Also note whether
the feature is upstream of or downstream from the reference location. Include as many water
management features as appropriate. Water management feature types are noted using the
following two-letter codes:

A stream reach (fig. 1) is the least clearly defined unit in the spatial
hierarchy (Frissell and others, 1986). Although a stream segment is a discrete
unit that should represent a uniform set of physical, chemical, and biological
conditions within a stream, its length (often more than several kilometers) prohibits effective
field collection of data. Therefore, the stream reach is chosen as the
principal sampling unit to collect physical, chemical, and biological
data. The type of reach characterizatio n conducted is dependent upon the
type of site (fixed or synoptic). However, the factors considered in
selecting a stream reach are the same for all sites.

The type and distribution of geomorphic channel units are the most
important factors in selecting a stream reach. Geomorphic channel units
are fluvial geomorphic descriptors
of channel shape and scour pattern that are widely used in habitat
assessment surveys (Orth, 1982; Ohio Environmental Protection Agency,
1989). The development of specific sequences of geomorphic channel units
is a fundamental stream process (Ying, 1971; Beschta and Platts, 1986).
Identification of geomorphic channel units is important because it
classifies stream habitat at a spatial scale relevant to most biota in
streams (Frissell and others, 1986). Three types of geomorphic channel
units are considered when selecting a stream reach--pools, riffles, and
runs (fig. 4).

The stream reach should include at least two examples
each of two types of geomorphic channel units. Only those geomorphic
channel units that are greater than 50 percent of the channel width are
considered when the length of the reach is determined. The